In recent years, wide attention has been drawn to studies of high-precision patterning in the manufacturing of high-performance micro-semiconductors, electron devices, biological functional materials, etc. Methods for realizing patterning include, but are not limited to, photo-etching, electron-beam etching, micro-contact transfer printing, and ink-jet printing. Conventional photo-etching, electron-beam etching, and micro-contact transfer printing requires complex manufacturing procedures and high costs, which limits their use in large-area patterning. Ink-jet printing technology makes it easier to realize direct writing of complex large-area patterns and patterning in functional composite materials, has simple manufacturing procedures, and is cost-effective, which makes ink-jet printing one of the most promising patterning methods. At present, ink-jet printing technology has been widely applied in the manufacturing of various kinds of functional devices. Ink-jet printing of high-precision patterns can realize accurate location of ink droplets, which is crucial to increase of resolution ratio of the micro-patterns formed and improvement on micro-device functions. Yet, due to restriction of the orifice diameter, the diameter of the smallest dot of or the most narrow line width of the patterns printed by the existing ordinary ink-jet printers, is only 20-30 μm. In the meanwhile, due to the “coffee ring effect” during the evaporation of ink droplets, functional solutes have different deposition density in the center of and on the edge of the ink droplets, which decreases uniformity of the patterns formed. These defects restrict, to a great extent, application of ink-jet printing technology in the manufacturing of high-performance micro-devices.
To further increase the utilization ratio of materials and decrease complexity of manufacturing procedures, great importance has been attached to ink-jet printing technology. The technical principle of ink-jet printing technology is as follows: hole-transporting materials, such as PEDOT/PSS (doped with polyaniline (PANI)), and solutions of RGB (three primary colors, red, green, and blue) light-emitting materials are respectively sprayed, through a micron-sized printing spray head, into the sub-pixel pits on the pre-patterned ITO (indium tin oxide) substrate, so as to form RGB light-emitting pixel units. This non-contact printing method, avoids contamination of functional solutions by contact, remarkably saves expensive light-emitting materials, and further greatly shortens the film-forming time through printing by means of a sprayer head with a plurality of jets (128 or 256). In conclusion, the ink-jet printing technology has significant advantages over the prior art in terms of saving raw materials and reducing costs.
With rapid development of the ink-jet printing technology, an increasing number of manufacturers are using it to manufacture organic light-emitting diodes (OLEDs) and organic light-emitting display devices.
An OLED device manufactured based on the ink-jet printing technology in the prior art comprises a substrate and a metal electrode, an ITO anode, a bank layer, an organic light-emitting layer, and a cathode which are successively formed on the substrate.
After the pixel pits are filled with droplets, the process of forming a film by drying can be explained by a technical term “coffee ring effect”, the principle of which is as follows. While the droplets are spreading on the substrate, due to reasons like surface defects, solutes will be affected by the “pinning effect” around the contact line and the droplets will remain in the spreading shape. Since the solvent around the contact line evaporates quickly, the solution will move from the droplet center to the droplet edge to compensate the evaporated solvent, due to which the solutes will deposit on the substrate and a non-uniform thin film with a center thinner than the edge will be formed (i.e. the coffee ring). The “coffee ring effect” will lead to non-uniform luminance of OLEDs.